A concept-formation study was run using sets of sentences in eight different syntactic patterns as target categories. These were based on all possible combinations of voice (active or passive), mood (declarative or interrogative), and modality (affirmative or negative). Subjects were 32 senior high school students who participated as volunteers in the computer-controlled experiment. Subjects were able to categorize sentences based solely on sentence types rather than semantic content, but an analysis of the errors committed in the course of learning showed that it was the semantic significance of different types rather than pattern differences as such to which a subject responded. The implications of this for grammatical formulations and the interpretation of psychological research were discussed.